Falling film heat exchangers usually include an array of vertical tubes. The tubes can be exposed or surrounded by a shell. The process liquid can be either inside or outside the tube with the heat exchange fluid on the other side.
Although falling film heat exchangers are used to heat a liquid feed stream, they can also be used for cooling such a stream. Falling film heat exchangers of the described types can be used as freeze exchangers for producing fresh water from brackish water and seawater, for concentrating fruit and vegetable juices, and industrial crystallization processes. See U.S. Pat. No. 4,286,436. As the liquid flows through each tube, it can be cooled enough to crystallize a solid from the liquid. Thus, by cooling seawater, ice is obtained which when separated, washed and melted provides potable water. When a fruit or vegetable juice is similarly chilled, ice forms and is removed to provide a concentrated juice.
Freeze exchangers of the described type often use as the cooling fluid a refrigerant such as ammonia, a Freon brand refrigerant, butane or propane.
For a substantial number of years, falling film shell and tube freezers which chill water and freeze it on the inside walls of the tubes by evaporation of a liquefied refrigerant in the shell have been commercially available. The product, often called tube or core ice, is used in restaurants, hotels and industries which need a continuous supply of ice. After a sufficient amount of ice is formed in the tubes, the liquefied refrigerant is drained from the shell and warm gases are admitted, causing the ice to melt on the surface adhering to the tube. The tube ice then slides out of the tube to a cutter which cuts it to desired length.
The efficiency of the described system of making tube ice, whether for ice-making itself or for concentrating aqueous liquids, is limited by flooding the shell with liquefied refrigerant because the liquefied refrigerant must be removed before the warm gas is introduced into the shell to melt and free the ice. Removing the refrigerant liquid and returning it later to repeat the ice forming step consumes power, increases the cost and slows ice production. It also inherently requires a large amount of refrigerant which is a costly component for very large systems. Additionally, using a warm gas to melt the ice surface is inherently inefficient since heat exchange between a gas and a solid is much lower than between a liquid and a solid.